Visual suppression may be characterized by a decrease in sensitivity or acuity in an eye. The decreased sensitivity may be caused by non-transmission, or poor transmission, of the optic nerve or from alterations in sensitivity in the brain. For example, deprivation of vision, strabismus (misaligned eyes), vision obstructing disorders, anisometropia or optic neuritis may cause visual suppression. There also can be unequal sensitivity between the two eyes because of changes in light transmission (e.g. a dense cataract) or a retinal abnormality in one eye.
A Worth 4-dot test is one conventional test that may be administered to a subject to determine if the subject is suppressing an eye. The Worth 4-dot test involves the use of a device with two (2) green lights, one (1) red light, and one (1) white light. A subject wears glasses having one green filter or lens over one eye (e.g., the left eye) and one red filter or lens over the other eye (e.g., the right eye). The subject is instructed to look at the white light and verbalize whether he or she can see all four dots. If, for example, the subject can see all four dots, then it may be concluded that neither eye of the subject is being suppressed. If, however, the subject can only see three lights (two green lights and the white light), then it may be concluded that the subject's right eye (i.e., the eye associated with the red filter) is being suppressed. Alternatively, if the subject can only see two lights (the red light and white light), then it may be concluded that the subject's left eye (i.e., the eye associated with the green filter) is being suppressed. Other clinical tests include assessment of the pupil and brightness sense assessment.
All of the current methods for assessing visual suppression have limitations. The Worth 4-dot test is only a qualitative test in that it can be used to determine whether an eye is being suppressed, but it cannot be used to determine the depth (i.e., the quantitative amount) of suppression. There must be a severe impairment in visual sensitivity before a change can be detected with the Worth 4-dot. Pupil assessment does not necessarily correlate with actual visual function. Brightness sense disparity has limitations because it requires only using one eye at a time that may not accurately assess vision with both eyes open. A system or apparatus that could be used to determine the depth of suppression would allow a medical professional to select appropriate visual therapy and determine the progress of the therapy by periodically re-testing the subject.